Ringorang Challenges are housed in a Series. It is possible to have a single Challenge in a Series. However, many Sponsors run multiple Challenges represented in a Series.
When a user joins a Challenge, they are automatically joined to the Series of which that Challenge is a part. When other Challenges are launched within that same Series, the user is automatically joined to them as well.
If a sponsor wants to enroll a Challenge with only new users and have no previous users automatically joined to it, then the sponsor must set up that Challenge in a different Series.
Users can read information about a given Series in the Series Details screen that invites the user to join or unsubscribe.
Related FAQ: What is a Challenge?